cmDepends: allow multiple dependees per depender

This patch is heavily inspired by Michael Wild.
The interfaces cmDepends::Write and cmDepends::WriteDependencies where
extended to allow multiple dependees (sources) per depender (object).
cmDepends::Write first collect all dependencies into a std::set before
passing it to cmDepends::WriteDependencies.
cmDependsC::WriteDependencies also first collects all explicit and
implicit dependencies into a std::set and only then writes
depend.{internal,make}. The implementation of cmDependsFortran simply
loops over all sources and proceeds as before, whereas the cmDependsJava
implementation is as trivial as before.
This is for preventing exponential growth of depend.{internal,make} in
the next commit which fixes dependency-vector erasure in
cmDepends::CheckDependencies.
Inspired-by: Michael Wild <themiwi@users.sourceforge.net>